1. Field of the Invention
The field of the invention relates to ophthalmic equipment and methods. In particular, the field of the invention relates to the analysis of image data collected by wavefront aberrometers.
2. Description of the Related Art
The human eye includes a cornea and a crystalline lens that are intended to focus light that enters the pupil of the eye onto the retina. However, the eye may exhibit various refractive errors which result in light not being properly focused upon the retina, and which may reduce visual acuity. Ocular aberrations can range from the relatively simple spherical and cylindrical errors that cause myopia, hyperopia, or regular astigmatism, to more complex refractive errors that can cause, for example, halos and starbursts in a person's vision.
Many interventions have been developed over the years to correct various ocular aberrations. These include spectacles, contact lenses, corneal refractive surgery, such as LASIK or corneal implants, and intraocular lenses (IOLs). The diagnosis and specification of sphero-cylindrical spectacles and contact lenses for treatment of myopia, hyperopia, and astigmatism are well-established. Some surgery-based techniques are not as predictable as may be desired but are still in wide-spread use and can yield good corrective results.
A wavefront aberrometer is an ophthalmic instrument capable of measuring ocular aberrations. These include low-order aberrations like defocus and the magnitude and axis of regular astigmatism. These low-order ocular aberrations can be quantified by, for example, second-order Zernike polynomials. In addition, wavefront aberrometers may also be capable of measuring higher-order aberrations of the patient's vision. These higher-order aberrations can be quantified by, for example, third-order, or higher, Zernike polynomials. These instruments can provide the theoretical information required to improve vision correction beyond the lower-order aberrations of defocus and regular astigmatism, such as, for example, in a LASIK procedure. However, wavefront aberrometers are also useful in diagnosing and prescribing spectacles, contact lenses, and IOLs, which typically correct sphero-cylindrical aberrations.
Several different types of wavefront aberrometers are known. These include Shack-Hartmann wavefront aberrometers and Talbot-Moiré wavefront aberrometers, among others. A Shack-Hartmann wavefront aberrometer operates by injecting a probe beam of laser light into the eye of a patient. The probe beam scatters from the retina back towards the instrument. The optical wavefronts of the scattered beam are aberrated by the patient's eye. After emerging from the eye, the scattered beam is collected and transmitted to an array of lenslets. The lenslets sample the beam at different spatial locations and focus it onto a detector in the form of an array of spots. If the scattered beam consists of planar wavefronts, then the array of spots focused on to the detector by the lenslets are regularly spaced. However, aberrated wavefronts cause each of the spots to be displaced in a manner that depends upon the local curvature of the wavefronts at the spatial location of each lenslet. An image of the spot pattern can be analyzed to determine the refractive properties of the patient's eye.
A Talbot-Moiré wavefront aberrometer also projects a probe beam of laser light into the eye of a patient. After the probe beam scatters from the retina, the scattered beam is collected and transmitted to one or more reticles, such as Ronchi grids. In one design, when the Ronchi grids are separated by a Talbot distance and are rotated with respect to one another, a Moiré fringe pattern is created, which can be imaged by a camera. An image of the Moiré fringe pattern can be analyzed to determine the refractive properties of a patient's eye.